Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen in the United States with an incidence of 4.5 million cases annually. The objectives of this project are to assess the frequency of C. trachomatis infections in selected populations, to develop and evaluate sensitive nucleic amplification assays for diagnosis, and to study the immunopathogenesis of C. trachomatis and C. pneumoniae infections. Using three new molecular amplification assays (PCR, LCR, and TMA), we screened over 2,000 male and 20,000 female patients in Baltimore. The overall prevalence of chlamydia ranged from 7% in family planning clinics to over 20% in adolescents. In a prospective study of 3,202 sexually active adolescents aged 12 to 19 years, chlamydia infection was found in 24% of first visits and in 14% of repeat visits; 29% of adolescent females have at least one positive test in a one-year period. Females aged 14 years had the highest age-specific chlamydia prevalence rate of 27.5%. The chlamydia incident rate was 28 cases per 1,000 person-months with a median time of 6 months to re-infection. With such high rates of infection, we recommend screening all sexually active adolescent females for chlamydia infection at least twice yearly regardless of prior infections, condom use from multiple partner risk. In a cost-effective analysis, we further documented that universal screening with non-invasive urine-based DNA amplification and use of self-administered vaginal swabs for other STDs offered the most cost-effective strategy for frequent screening of STDs, particularly in adolescents. Between January 1996 and December 1997, we screened by LCR for C. trachomatis urine in 13,204 new female U.S. Army recruits from 50 states and four territories. The overall prevalence was 9% with a peak of 12% in 17-year-old females. Recent vaginal sex, young age (< 25), being African American, having more than one sex partner, and inconsistent condom use were independently associated with chlamydia infection. In this national survey, we documented that the prevalence of chlamydia infection was very high in female military recruits and that all recruits should be screened and treated for chlamydia in order to reduce infection, transmission, and sequelae. Internationally, in a community-based trial of mass STD treatment in the Rakai district of Uganda, we documented relatively high rates of chlamydia and gonorrhea in 15-24 year-olds (5%). Ten months after mass antibiotic treatment, there was a 50% reduction in both chlamydia and gonococcal infection. This study demonstrates that the vast majority of STD infections are asymptomatic (80%), and that mass antibiotic treatment is effective in lowering infection rates. Future studies will address the effectiveness of mass treatment in limiting HIV transmission. In order to provide a cost savings for molecular amplification screening of urine in developing countries, we developed a pooling algorithm in which urine from ten individuals can be pooled and analyzed by molecular amplification for C. trachomatis or N. gonorrhoeae. The pooling algorithm demonstrated 59% reduction in costs and was sensitive and specific compared to testing individual samples. We recently demonstrated a direct correlation between C. pneumoniae and atherosclerotic heart disease by the recent isolation of C. pneumoniae in an explanted heart with severe coronary atherosclerosis. C. pneumoniae has also been demonstrated in nearly 50% of coronary and carotid atheromas by immunocytochemistry and/or PCR. In vitro studies have demonstrated that C. pneumoniae can infect and proliferate in coronary artery endothelial cells and aortic artery smooth muscle cells. In preliminary animal studies, C. pneumoniae-inoculated mice appeared to have an increased rate of atheroma formation in the carotid artery and aortic artery compared to control animals, suggesting an etiologic or co-factor role in the development of atherogenesis.